40 THE BIOLOGY OF FLOWERING PLANTS 



tap, show the same type and extent of root system under 

 widely different conditions of moisture supply ; they are 

 profoundly conservative. 



The relation of water supply to root development may 

 be illustrated by the behaviour of three of the cereals ex- 

 amined in the prairies and the short-grass plains as shown 

 in Table VI. 



TABLE VI 

 Root Systems of Cereals in Different Environments 



The rainfall on the plains, and the water supply in the 

 soil, are much less favourable than on the prairies ; the 

 unfavourable conditions are reflected in a poorer shoot 

 growth as well as in the less well-developed root systems. 



3. Mineral Nutrients. — The action of the supply of 

 mineral salts is peculiarly difficult to disentangle from other 

 effects, not only because it is closely related to other soil 

 conditions, and in particular to moisture content and 

 reaction, but also because of secondary reactions through 

 the effect on the shoot system. Nobbe (1862), who grew 

 plants in soil, alternate layers of which had been watered 

 with nutrient solution, found branching of the root system 

 more vigorous in these. Weaver, Jean, and Crist (1922) 

 used a similar method, but isolated the layers of soil 

 fertilised with nitrate by thin layers of wax. They found 

 that the roots of the potato and of various cereals always 

 branched more vigorously in the fertilised layer. Frank 

 (1893) found that the half of a pea root system grown in 

 a soil rich in nitrates developed better than the other half 

 in poor soil. Here the effect of the shoot is ehminated, 



